Light and Reflection

El método, paso a paso

1. 1

   Draw the normal first

   a dashed line at 90° to the surface where the ray hits

   Every angle in this topic is measured from the normal, never from the surface. Drawing it before any angle is measured is what makes the law of reflection produce the right numbers.

2. 2

   Apply the law of reflection

   angle of incidence = angle of reflection

   The incoming and outgoing angles are equal, measured from the normal. This holds for every reflecting surface, and it is why a mirror image appears the same distance behind the mirror as the object is in front.

3. 3

   Explain refraction through a change in speed

   light slows entering glass → bends towards the normal

   Light travels more slowly in denser materials. Entering a denser medium bends the ray towards the normal; leaving it bends the ray away. This is why a straw in a glass of water appears broken at the surface.

4. 4

   Distinguish specular from diffuse reflection

   smooth surface → clear image · rough surface → scattered light

   The law of reflection holds for both. On a rough surface the normals point in many directions, so parallel rays scatter and no image forms. This is why paper is visible but shows no reflection.

5. 5

   Get the direction of seeing right

   light travels from the object to the eye

   We see because light enters the eye, not because the eye sends anything out. Ray diagrams must show arrows pointing towards the eye, and reversing them loses marks even where the geometry is correct.

Everything is measured from the normal

The normal is an imaginary line at right angles to the surface at the point where the ray strikes. Every angle in reflection and refraction is measured from it, and measuring from the surface instead gives an angle that is wrong by exactly ninety degrees.

Because this is the most frequent source of wrong answers, drawing the normal should be the first mark on any ray diagram. It costs a dashed line and it prevents the error entirely.

• Light travels in straight lines
• Angles are measured from the normal, not the surface
• Reflection: angle in = angle out
• Refraction: speed changes, so direction changes
• We see when light enters the eye

The misconception about seeing

A surprising number of students believe that seeing involves something leaving the eye — a belief with a long history and no basis. Light travels from a source, reflects off an object, and enters the eye. Nothing comes out.

This matters practically because ray diagrams require arrows in the correct direction. A diagram with rays leaving the eye describes an impossible process and is marked wrong regardless of how neat the geometry is.

Why light bends

Light travels at different speeds in different materials — fastest in a vacuum, slower in water, slower still in glass. When a ray meets a boundary at an angle, one side of the beam slows before the other, and the beam swings round.

The wheel analogy is useful: a trolley rolling at an angle onto sand slows on one side first and turns. Refraction is that, applied to a wave rather than a wheel.

How we teach light

We require the normal on every diagram before any angle is measured or labelled. It is a mechanical habit that removes the single largest source of error in the topic.

We also check the direction of the arrows on every ray diagram. The eye misconception is common enough that it is worth testing for directly rather than assuming it has been left behind in primary school.